The present disclosure relates to a sheet post-processing device that performs post-processing on a sheet on which image formation has been performed with an image forming apparatus such as a copying machine or a printer and a sheet post-processing method.
As this type of sheet post-processing device, a device is known that collects sheets in a stack in which an image is formed and that performs post-processing on the stack of sheets. For example, in a conventional sheet post-processing device, as an example of the post-processing, stapling processing is performed in which one ends of the stack of sheets are aligned and bound. The sheet post-processing device includes a case member that has a paper feed port and a paper discharge port. Within the case member, a main transport path and a post-processing transport path are provided. The main transport path receives a sheet supplied from the paper feed port and guides it to the paper discharge port. One end portion of the post-processing transport path is connected to a downstream-side end portion of the main transport path. At the other end portion of the post-processing transport path, a staple unit is provided. In a connection portion of the main transport path to the post-processing transport path, a drive roller that is driven by a motor is provided. On the side of the drive roller, a rotation alignment member is provided. On the lower side of the drive roller, a sheet support unit is provided. In a position of the sheet support unit opposite the drive roller, a driven roller is rotatably provided. The driven roller can be moved to a nip position where a nip portion is formed between the drive roller and the driven roller and to a maximum separate position that is separate a predetermined distance from the driven roller.
In the alignment of sheets, the drive roller is first driven by a drive means to the maximum separate position. In this state, the sheet is transported from the main transport path onto the sheet support unit. Here, the drive roller is separate from the driven roller. The sheet transported onto the support unit is supplied by the rotation alignment member to the post-processing transport path and is guided to the staple unit. Then, the back edge of the sheets makes contact with a reference plate portion of the staple unit, and thus the sheets are aligned. On the sheets collected in a stack after the completion of the alignment of the sheets, stapling processing is performed by the staple unit.
After the performance of the stapling processing, the drive roller is driven by the drive means from the maximum separate position to the nip position. In this state, the drive roller is rotated in a positive rotation direction, and thus the stack of sheets is discharged to a paper discharge tray. The paper discharge tray is connected to a lower side with respect to the paper discharge port on the outer wall surface of the post-processing device. The paper discharge tray is inclined toward the upper side from an upstream side to a downstream side in a paper discharge direction.
In the conventional sheet post-processing device described above, the front end portion of the sheet transported onto the support unit (on a post-processing tray) protrudes from the paper discharge port to the side of the paper discharge tray. Here, disadvantageously, for example, when the front end portion of the sheet is curled downward, the front end portion of the sheet hits the paper discharge tray and moves to a base end side (the upstream side of the paper discharge tray in the paper discharge direction), and thus the curling of the sheet further proceeds or the front end portion of the sheet is kept pressed to the paper discharge tray and is prevented from being moved in the paper discharge direction, with the result that the front end portion of the sheet is swelled upward. Consequently, disadvantageously, the sheet is excessively curled, and thus a paper jam occurs when the sheet is transported by the rotation alignment member to the post-processing transport path, and a failure occurs in the alignment of the sheets.